1. Field of the Invention
This invention relates to the field of transistor temperature sensors, and particularly to methods of reducing measurement errors due to intrinsic base and emitter resistances in such sensors.
2. Description of the Related Art
Numerous circuit devices, such as transistors, diodes and resistors, have operating characteristics that are temperature dependent. Because of their temperature dependencies, such devices are extensively used as temperature sensors. For example, germanium and silicon diodes can be operated at a constant forward-biased current, and the resulting forward-biased voltage measured to determine the temperature in accordance with the standard forward-biased diode equation:
V=kT/q ln I/Is
where V is the forward-biased voltage, k is Boltzmann""s constant, q is the electron charge, T is the absolute temperature in degrees Kelvin, I is the forward-biased current and Is is the diode""s saturation current.
In practice, the measurement of temperature with a diode is subject to several inaccuracies. The precise voltage-temperature relationship depends upon the actual details of the junction, notably the doping densities on either side of the junction, the dopant profiles and the junction area, as well as secondary considerations such as bulk and surface defects in the material. These factors are difficult to quantify with certainty, and many of the parameters in the device equations (such as mobility) are themselves temperature-dependent. Other effects, such as conductivity modulation and series resistances, can also complicate the device""s behavior.
Another approach employs two separate junctions which are fabricated on the same substrate, but which are operated at different current densities. This eliminates the effects of variations in doping levels and in the value of the bandgap voltage. The dual junction approach has been implemented with a pair of bipolar transistors whose emitter areas are in the ratio A. The difference in collector current densities gives rise to a difference in the base-emitter voltages for the two transistors. The relationship between the base-emitter voltage differential (xcex94Vbe) and the device temperature is given by the expression:
xcex94Vbe=kT/q ln A
While this approach offers significant advantages over the single junction temperature measurement, it still has some limitations. There is a certain amount of tolerance in the transistor fabrication, which introduces an ambiguity into the emitter area ratio. Furthermore, the accuracy of the equation is reduced by ohmic resistances associated with the junction, specifically the base resistance rb and the emitter resistance re. The base and emitter resistances may be considered to include both the intrinsic resistances inherent in the device, and the resistances associated with connecting lines. Calibration of such a sensor is required for most applications, and the fact that at least a pair of junctions are required introduces the possibility that differential strain across the substrate could result in poor tracking of junction voltages with a consequent error in the small xcex94Vbe voltage.
Another technique is described in U.S. Pat. No. 5,195,827 to Audy et al. Here, a single bipolar transistor is sequentially driven with three different currents, inducing three base-emitter voltages which are measured and used to calculate temperature. This approach also has significant shortcomings, however. Using three currents requires that the ratios between the currents be kept small, in order to avoid heating up the sensing transistor and thereby introducing error into the temperature measurement. Also, the calculations necessitated by a three-current approach are likely to require non-integer math, which can be difficult and/or impractical to implement.
A four current transistor temperature sensor and method are presented which overcome the problems noted above. The invention allows the use of large current ratios and simple temperature calculations, while still reducing or eliminating intrinsic base and emitter resistance errors.
A p-n junction, preferably the base-emitter junction of a bipolar transistor, is driven with four different currents in a predetermined sequence. Each of the four currents induces a respective base-emitter voltage, which is measured. The temperature of the transistor is calculated based on the values of the four driving currents and the four measured base-emitter voltages.
In a preferred embodiment, the four driving currents (I1, I2, I3 and I4) are arranged such that I1=n*I3, I2=n*I4, I1/I2=A and I3/I4=A, where A is a predetermined current ratio. In operation, I1 and I2 produce respective base-emitter voltages which are subtracted from each other to produce xcex94Vbe1, and I3 and I4 produce respective base-emitter voltages which are subtracted from each other to produce xcex94Vbe2. When so arranged, the difference between xcex94Vbe1 and xcex94Vbe2 is entirely due to the effect of series base and emitter resistances rb and re. The xcex94Vbe1xe2x88x92xcex94Vbe2 value thus provides a correction factor which enables temperature measurement errors due to rb and re to be eliminated. This arrangement also allows the use of large currents ratios, and greatly simplifies the calculations required to determine temperature T.